Electric accumulators are already known comprising an anode compartment containing a metal anode and a cathode compartment containing an electron acceptor substance, these compartments being separated from each other by a wall impervious to fluids and formed from a solid mineral electrolyte capable of allowing selective migration of the anode metal in the form of cations, when a potential difference is created between the anode and cathode.
French Pat. No. 1490198 describes an electric accumulator comprising an anode formed from an alkaline metal, preferably sodium, potassium or lithium, and an electron acceptor substance contained in the cathode compartment, and consisting of an alkaline polysulphide, preferably sodium polysulphide with a composition variable from Na.sub.2 S.sub.5 to Na.sub.2 S.sub.3 according to the state of charge of the accumulator. The solid electrolyte used in this accumulator for allowing migration of the cations of the alkaline metal is sodium beta aluminate, the approximate composition of which may be expressed by the formula Na.sub.2 O. 11 Al.sub.2 O.sub.3, or analogous compounds derived from sodium beta aluminate by at least partially replacing the sodium by another alkaline metal and/or by adding a mineral compound such as boron oxide B.sub.2 O.sub.3.
French Pat. No. 1491674 describes an electric accumulator of the same type as that which forms the object of the French Pat. No. 1490198, but in which the solid electrolyte used is not only beta aluminate but also a glass which conducts the cations of the alkaline metal of the anode, this glass being obtained either from a ternary mixture of sodium oxide Na.sub.2 O, aluminum oxide Al.sub.2 O.sub.3 and silica SiO.sub.2, or from a ternary mixture of sodium oxide, aluminium oxide and boron oxide B.sub.2 O.sub.3.
Accumulators of the type described in the two said French Patents have the considerable advantage relative to traditional lead accumulators (in which the energy density is of the order of 23 Wh/kg) of providing an energy density greater 100 Wh/kg.
However these accumulators suffer from the disadvantage of not being able to operate at a temperature lower than about 300.degree. C because of the need to maintain the electron acceptor substance (alkaline polysulphide or an alkaline polysulphide mixture) in the liquid state, its melting point, which depends on the state of change of the accumulator, usually varying between 265.degree. and 350.degree. C. Moreover, the high corrosivity of the alkaline polysulphides used in these accumulators causes rapid deterioration of the constituent parts thereof, which means that such accumulators have an insufficient life to enable them to be used industrially.
The fact that the wall separating the anode compartment from the cathode compartment is directly in contact with the alkaline metal of the anode is also prejudicial to the proper operation of such an accumulator, as this can cause short circuits and fractures in said wall.
Moreover, sulphur may be produced in an elementary and therefore electrically insulating and electrically uncharged form when the accumulator is charged or discharged rapidly, this likewise unfavourably influencing the operation of the accumulator.
The U.S. Pat. No. 3,877,984 describes an electric accumulator similar to those forming the object of the two aforesaid French Patents, and comprising, as in the case of these latter, an anode formed from an alkaline metal, and a solid electrolyte which allows selective migration of this metal in the form of cations, the electrolyte being in the form of an impermeable separable wall between the anode compartment and cathode compartment.
The electron acceptor substance contained in the cathode compartment of this accumulator is a metal chloride such as antimony chloride, cuprous chloride, ferric chloride or nickel chloride. When the accumulator is in its state of maximum charge, this chloride is at least partly dissolved in an electrolyte, which is liquid at the operating temperature of the accumulator, and consisting of an alkaline chloroaluminate corresponding to the anode metal, for example sodium chloro-aluminate NaAlCl.sub.4.
The operating temperature range for this accumulator lies between about 180.degree. and 200.degree. C, these limits being lower than those of the accumulator described in French Pat. Nos. 1490198 and 1491674 because of the use of said alkaline chloro-aluminate, the melting point of which lies between 125.degree. and 200.degree. C, as the liquid electrolyte in the cathode compartment.
Swiss Pat. No. 517385, with which in particular the U.S. Pat. No. 3,370,771 and the Federal German Republic Pat. No. 2051395 correspond, describes an electric accumulator comprising an anode compartment containing a metal anode formed from a metal chosen from the alkaline and alkaline earth metals, and a cathode compartment containing an electron acceptor cathode in the form of a solid member comprising an electron acceptor substance consisting preferably of a metal salt, and in particular of a transition metal halide, said cathode compartment also containing a solution of at least one salt of the anode metal in a polar organic solvent such as dimethylformamide, N,N'-dimethylacetamide, gamma-butyrolacetone, tetrahydrofuran or propylene carbonate, the anode compartment being separated from the cathode compartment by an impervious wall of a solid electrolyte which allows the selective migration of the cations of the anode metal.
In one particular embodiment of this accumulator, the anode compartment contains a solution of at least one salt of the anode metal in a polar organic solvent, identical or similar to the solution contained in the cathode compartment, this solution being interposed between the anode and the separation wall.
This accumulator has the advantage of being able to operate over a temperature range which includes ambient temperature.
However, although such an accumulator may have a high energy density, for example of the order of 250 Wh/kg, its power density is limited because of the relatively low electrical conductivity of the organic solutions which can be used in the cathode compartment (and also in the anode compartment if the case arises).
Consequently, this accumulator appears particularly suitable for use in fields in which a high power density is not necessary, but not suitable for use in fields which on the contrary require high power densitites, such as for supplying electric traction motors for motor vehicles.